1. Field of the Invention
The present invention pertains to liquid crystal on silicon (LCOS) displays, and more particularly to improved temperature control and compensation method for the microdisplay systems.
2. Description of the Prior Art
Since microdisplay systems, especially the liquid crystal on silicon (LCOS) Microdisplay frequently operate in the hot interior of a projection device, the microdisplay technology is still challenged by the need to effectively control and compensate the performance variations caused by temperature increases such that the quality of display would not be impaired by uncontrolled high temperatures. There are several prior art approaches taken to solve this well-known problem. A first one was reported by Kurogane et al “Reflective AMLCD for Projection Displays”(D-LIA. Paper 5.3, Proceedings SID 1998) is the use of an electro-optic mode that does not exhibit noticeable thermal variation in the linear region of interest. However, the availability of the materials employed and special manufacture processes and mode of operations would significantly restrict the usefulness of the proposed microdisplay systems. Another is the approach taken in U.S. Pat. No. RE 37,056, Wortel, et al, “Temperature Compensated Color LCD Projector” (Feb. 20, 2001). where the inventors disclose a method to manufacture the cell in such a manner that the slopes of the electro-optic curves measured at different temperatures in the same liquid crystal device are quite close. A simple temperature measurement system is employed to provide information to a system that can adjust the column drive voltage and thus effect the compensation. However, this particular approach is of limited usefulness because the method requires a very specific approach to the design and manufacture of the cell.
In view of the current state of the art of microdisplay temperature control, there is an ever-increasing demand for new methods and system configurations that can effectively control the temperature and to compensate the performance variations caused by the temperature changes due to the temperature sensitivities of the microdisplay systems. There are several reasons for such increased demand. First, it is observed from operations of microdisplay systems that a liquid crystal experiences a rise in temperature from ambient over a period of 20 to 30 minutes after a system is turned on. This rise in temperature is attributable in part to a rise in ambient temperature within the product case due to heating of the air within by such items as the lamp and by other electronic components. A second major source of heating is the heat generated from the thermal characteristics of the silicon in the LCOS microdisplay itself. A third major source is heat caused by the illumination from the lamp falling on the microdisplay itself. The degree of temperature increase depends on the thermal design of the product and the environment in which it operates. A second reason for the increasing demand to control and compensate temperature effect for a microdisplay system is an observation that the system performance of a microdisplay is strongly temperature dependent. A first sensitivity of LCOS microdisplays is the reduction of the birefringence of the liquid crystal material with elevated temperature within such a display with thus the electro-optic (EO) curve for such a device is highly temperature dependent. One particular aspect of this temperature driven effect is that the dark state rises as temperature deviates from the design temperature and therefore the contrast of such a system suffers.
FIG. 1A shows the strong influence of the temperature changes on the electro-optic performance of a nematic liquid crystal cell constructed by using a 45° twisted nematic (45° TN) in normally black (NB) electro-optic mode. The cell is nominally 5.5 μm thick. The clearing temperature of the liquid crystal is not precisely known but is estimated to be 85° C. Four sample temperature curves determined by experiment are depicted. Thus the major effects of the temperature variations are clear upon inspection. First, the liquid crystal (LC) curve shifts to lower voltage as the temperature of the LC rises. Second, the intensity of the achievable dark state rises as temperature rises. The apparent magnitude of the dark state intensity appears to increase nonlinearly as temperature rises. Third, the location of the peak of the voltage curves shifts to lower voltages as the temperature rises. Fourth, the height of the peak of the voltage curve drops slightly as temperature rises. Finally, the voltage required to achieve the best dark state does not appear to move significantly with changes in temperature.
Referring to the LC curves of FIGS. 1B and 1C disclosed in U.S. Pat. No. RE 37,056 for further understanding of the temperature dependence of the performance of a microdisplay system. FIG. 1B shows diagrammatically transmission/voltage characteristics of a display device according to the invention at different temperatures, while FIG. 1C shows similar characteristics for a conventional display device. The data as illustrated in FIGS. 1B and 1C are curves for normally white mode transmissive displays which are also representative of reflective mode normally white displays as well. As disclosed in the patent, FIG. 1B presents data that is better behaved than that of FIG. 1C. Implicit in the patent itself in describing the difficulty is the likelihood that the liquid crystal cell is being driven by an analog drive source, such as a Digital-to-Analog Converter (DAC). The DAC would have to be adjusted to a completely different slope and origin in configuring it to drive at different temperature in the case of FIG. 1C. The control and compensation of temperature variation for microdisplay system according to the disclosed techniques would become more cumbersome and inconvenient due to this adjustment requirement.
Thus from the above it is clear that temperature is an important factor in the performance of a liquid crystal device. It is also clear that knowledge of the temperature of a liquid crystal device can enable several commonly known control mechanisms in the electro-optical-mechanical design of a product using such devices. In order to control the microdisplay operational temperature, traditional measures includes the use of fan controlled by a thermostat for activating a fan to increase the air circulation of a microdisplay system. Alternatively the thermostat may be position to measure the heat at a set of heat sinks mounted to the back of the microdisplays. Additionally, the knowledge of several control mechanisms in the electro-optical-mechanical design embodied in different products using such mechanisms can be implemented to further exploit such knowledge to achieve optimal performance. However, as of now, the conventional technologies in microdisplay temperature control still have not fully take advantage of the availability of different control mechanisms to improve and enhance the temperature control and compensation for microdisplay systems operated under widely varying temperatures.
For these reasons, there is still need in the art of microdisplay such as the liquid crystal on silicon (LCOS) display to provide improved system architecture and methods of temperature control and compensation to improve the system performance under wide ranges of temperature variations such that the above-mentioned limitations and difficulties can be overcome.